The present invention is related to coated panels, and, in particular, to a formaldehyde-free coating that is applied to a major exterior surface of a fibrous panel to resist sag.
Fibrous substrates are light weight, porous composite materials which are used for many different purposes including panels in suspended ceiling systems. They are produced from a water based slurry mixture containing fibers, a binding system and other additives. Fibers, which are typically used as reinforcing materials, include mineral wool, glass and cellulosic fibers. Binding systems, which hold the fibers and other additives together, include starches, latex, reconstituted paper products and other polymeric materials. Other additives include fillers such as expanded perlite, clay, etc.
It is widely known in the art that varying the material percentages of the aforementioned components ultimately impacts the physical and mechanical properties of the fibrous substrate, particularly when the fibrous substrate is installed in a horizontally extending suspended ceiling system. For example, it is well known to those skilled in the art that after installation into a suspended ceiling framework, a fibrous substrate has a tendency to sag when exposed to high humidity environment due to the hydrophilic nature of certain components of the core substrate, e.g. water soluble polymer binders (e.g. starch) and cellulosic fibers (e.g. newsprint). More specifically, after absorbing moisture, the substrate loses its modulus and sags by its gravity. Thus, conventional wisdom is that as the material percentages of these hydrophilic components are increased, the sag performance of the substrate decreases.
There have been various attempts to improve or even eliminate sag in these conventional fibrous substrates. One known method is to apply a hygroscopic coating on one of the major exterior surfaces of the substrate. More specifically, such coating includes a binder system which is hydrophilic and capable of absorbing moisture with rising humidity and desorbing moisture with decreasing humidity. Thus, when humidity rises, the hygroscopic coating absorbs moisture and expands in its volume and dimension, thus creating an expansion force on the surface of the substrate to which the coating is applied. In essence, the coating counter-acts the compressive force caused by the humid conditions.
In order to resist the compressive force from the underlying fibrous acoustic panel at high relative humidity, not only must the back coating be hygroscopic and create an expansion force, it is further required for the coating to maintain a high modulus. It is well understood in the art that polymer modifications are necessary for the hydrophilic polymer to maintain a high modulus after absorbing high level of moisture. One known method of polymer modification is by means of crosslinking. Once the polymer is properly crosslinked, the polymer matrix expansion will be limited, and, in turn, the polymer softening, i.e. loss of modulus, at high humidity conditions will be minimized.
Additionally, there are several known formaldehyde-free compositions for use as binders for making fibrous substrates. For example, U.S. Pat. Nos. 6,221,973 and 6,331,350 describe a formaldehyde-free fiberglass binder including a polyacid, such as polyacrylic acid, and a polyol, such as glycerol, diethanolamine, triethanolamine, sorbitol, or ethylene glycol. However, the main drawback of these formaldehyde-free binder solutions is their low pH which is often not compatible with other coatings and/or causes corrosion of processing equipment.
Thus, what is needed is an improved coated fibrous substrate which: does not emit environmental irritants such as formaldehyde; is sag resistant while at the same time maintains a high modulus; is compatible with other coatings and fillers; and avoids corroding processing equipment